These
graphs show an example of a recording taken during the Vendée Globe on EBP‑Gartmore
(on the 7th of January at 12:12 UTC, position 49°16.8’S, 147°55.2’W).
To keep it readable, only the vertical acceleration at the bow (« hvb », red), the lateral acceleration at the centre
of gravity (« swc », blue) and the vertical acceleration at
the COG (« hvc »,
violet) are shown here.
The
units for the vertical axis are « gs », while time in seconds is shown on the horizontal
axis. Four wave encounters can be seen in this case, the third of which produced
a peak acceleration of 2.93g at the bow and of 0.83g at the COG (taking into
account all three components). At the time of the “slam” the boat was sailing
at 11.2 knots (with a true wind speed of 21.4 kts and a true wind angle of
72 degrees) with a heel angle of 11 degrees. Immediately after the acceleration
peak, oscillations of the vertical components can be observed. These are due
to structural vibrations of the hull and rig, which are excited by the transient
loading on the hull. Through filtering, we can isolate this component of vibration
and, through integration, we can estimate the amplitude of the deflection
associated with it. Finally, if we use the method proposed by Allen and Jones
(“A simplified method for determining structural design limit pressures on
high performance marine vehicles”, American Institute of Aeronautics and Astronautics,
1978), we may deduce that this “slam” is likely to have produced and average
pressure of 0,18 bar on the most heavily loaded hull panels. This value is
considerably lower than those for which the structure is designed. However,
this “slam” is far from being the heaviest recorded. In general, the results
we have obtained until now tend to confirm the validity of the choices made
in the design of the analysed structures.